Update pytest from 3.8.2 to 3.9.1 (#762)

Author: pyup-bot

requirements.txt

@@ -11,7 +11,7 @@ pymongo==3.7.2
 pytest-cover==3.0.0
 pytest-randomly==1.2.3
 pytest-tornado==0.5.0
-pytest==3.8.2
+pytest==3.9.1
 python-dateutil==2.7.3
 pyyaml==3.13
 six==1.11.0